minor clean-ups (#1600)

- remove obsolete optimisation in `AST_Binary` after #1477
- improve `TreeWalker.has_directive()` readability and resilience against multiple visits

lib/compress.js

@@ -1664,7 +1664,7 @@ merge(Compressor.prototype, {
     /* -----[ optimizers ]----- */
 
     OPT(AST_Directive, function(self, compressor){
-        if (compressor.has_directive(self.value) === "up") {
+        if (compressor.has_directive(self.value) !== self) {
             return make_node(AST_EmptyStatement, self);
         }
         return self;
@@ -3018,16 +3018,6 @@ merge(Compressor.prototype, {
     var commutativeOperators = makePredicate("== === != !== * & | ^");
 
     OPT(AST_Binary, function(self, compressor){
-        var lhs = self.left.evaluate(compressor);
-        var rhs = self.right.evaluate(compressor);
-        if (lhs.length > 1 && lhs[0].is_constant() !== self.left.is_constant()
-            || rhs.length > 1 && rhs[0].is_constant() !== self.right.is_constant()) {
-            return make_node(AST_Binary, self, {
-                operator: self.operator,
-                left: lhs[0],
-                right: rhs[0]
-            }).optimize(compressor);
-        }
         function reversible() {
             return self.left instanceof AST_Constant
                 || self.right instanceof AST_Constant